Waterproof flooring: what the word covers, and what it leaves out

Waterproof describes the plank, not the floor

A waterproof plank is one that water does not swell or delaminate, and a floor made of those planks can still let water through to the subfloor.

Rigid core vinyl plank is made of materials that do not absorb water. Leave one in a bucket overnight and it comes out the same size. That is what the word on the box is describing, and it is accurate.

A floor is more than its planks. Click together planks meet at joints that are tight and not sealed. A spill wiped up in a minute never gets through. A dishwasher that leaks for a week sends water through the joints and around the edges of the room, where the floor stops short of the wall to leave room for expansion.

Once water is under a floating floor it stays there, because the same waterproof planks that kept it out from above now keep it from evaporating. The planks are fine. The subfloor is wet, and it stays wet.

Water from above and water from below

Spills and leaks come from above and are occasional, while moisture from a concrete slab comes from below and is constant, and the two need different defenses.

Kitchens, bathrooms and laundry rooms have a from above problem. The risk is an event: an overflowing sink, a failed supply line, a pet bowl. A floor that tolerates standing water for a few hours covers most of it.

Basements and slab on grade rooms have a from below problem. Concrete is porous, and ground moisture moves up through it as vapor all year. Nothing is visibly wet. The moisture collects under whatever is laid on top, and if that material is wood or wood based, it absorbs it.

This is why solid hardwood does not go over concrete or below grade, and why even engineered wood goes over a slab only after a moisture test. It is also why a waterproof floor laid over a damp slab without a vapor barrier can grow mold underneath while looking perfect from above.

The test a slab needs before any floor goes on it

A concrete slab is tested for moisture with a method that produces a number, and that number decides which floors and which adhesives are allowed.

The standard method for measuring moisture inside a slab is ASTM F2170, which uses probes set into drilled holes to read the relative humidity within the concrete. It is a measurement taken over time, with the room at its normal temperature, and it reflects what the floor will actually live on.

Flooring and adhesive manufacturers each publish the highest reading their product tolerates. A slab that reads under the limit can take a glued floor. One that reads over it needs a moisture barrier first, a floating floor with a suitable underlayment, or a different product.

A new house is not exempt. Concrete keeps releasing water for a long time after it is poured, and a slab in a house that was finished last year can read higher than one from 1960. We test before recommending anything for a slab.

Vinyl plank, laminate, engineered and solid, side by side

Vinyl plank handles water from above and tolerates a slab, laminate resists spills but swells when soaked, engineered wood works over a dry slab, and solid hardwood needs a wood subfloor above grade.

Laminate deserves a specific note, because it is often sold with similar language. Its surface resists water well. Its core is fiberboard, and water that reaches the core through a joint or a cut edge makes it swell, and it does not shrink back. Water resistant laminate buys time to clean up a spill. It is not a basement floor.

Engineered wood is real wood on a plywood core, built to stay flat where solid wood would cup. Over a slab that has tested dry it performs well and can be sanded later if the wear layer is thick enough. It is still wood, and it does not tolerate standing water.

Solid hardwood is the floor that lasts longest and can be refinished most often. It belongs on a wood subfloor, above grade. In a kitchen it does well for decades when leaks are caught promptly, which is how most kitchens in older houses in this region were floored.

Flatness matters as much as moisture

A click together floor depends on the subfloor being flat, and most failures blamed on the product are joints that broke over a dip.

The locking edges of a plank are thin. Over a flat base they stay engaged for years. Over a low spot the floor flexes each time someone walks across it, and the joint eventually opens or snaps. From above it looks like a defective plank.

So the preparation is most of the job. We check the subfloor with a long straightedge, fill the low areas with leveling compound and grind or sand down the high ones. In an old house, where floors have settled for a century, this can take longer than laying the planks.

It is also the main reason quotes for the same room differ. One includes the flattening and one does not, and the difference shows up a year later.

What is already on the floor in an older house

Old sheet flooring, tile and the adhesive under them can contain asbestos in housing from before the 1980s, so they are left undisturbed or tested before removal.

Kitchens and basements in older houses often carry several layers of earlier flooring. Resilient tile and sheet goods from the middle of the twentieth century, and the black adhesive used under them, were commonly made with asbestos. It is not a hazard while it stays intact. Sanding, scraping or breaking it up is what releases fibers.

The practical options are to leave the old layer in place and install over it, when it is flat and well bonded, or to have a sample tested by a laboratory before any removal. If it tests positive, removal is work for a certified asbestos abatement contractor and not for a flooring crew.

This applies to a large part of our service area. In the dataset we publish at /data/, approximately 60.5 percent of 3,899,409 housing units predate 1978, and in Philadelphia approximately 82.8 percent do.

Which rooms actually need a waterproof floor

Basements, full bathrooms and laundry rooms are where a waterproof plank earns its place, and kitchens and powder rooms can take wood when the owner prefers it.

A basement is below grade and usually on a slab, and it is the room most likely to take water from an event nobody controls: a failed sump pump, a storm, a water heater. Vinyl plank over a tested slab is the sensible floor there.

A full bathroom has standing water as a routine event. A laundry room has two supply lines and a drain. Both are reasonable places for vinyl or tile.

A kitchen is a judgment call. If the rest of the main floor is hardwood, continuing it through the kitchen gives one surface with no transition, and it can be refinished with the rest. The risk is a slow appliance leak, and a leak detector under the dishwasher and the sink is an inexpensive answer to that.

How this plays out across the region

The right floor follows the age of the house: older houses have wood subfloors and damp basements, and newer ones have slabs and open plans.

In the 635 places in our data at /data/, 113 have a median home built before 1950. Houses there typically have wood framed floors over a basement with a slab that was never designed to be finished space. Upstairs, solid hardwood is the natural choice. In the basement, it is vinyl plank over a tested slab, or nothing until the dampness is dealt with.

In the 216 places where the median home dates from 1980 or later, ground floors are more often slab on grade. Engineered wood and vinyl plank are the two candidates, and the moisture test chooses between them.

Either way the decision is made by looking under the room. We do that at the estimate, which is free, and we tell you when the floor you had in mind is the wrong one for what is underneath.